Yes, RUNNER is a Scrabble word!

Letter values in Scrabble US: R 1, U 1, N 1, N 1, E 1, R 1.

Is it a valid word?

RUNNER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

someone who imports or exports without paying duties
someone who travels on foot by running
a person who is employed to deliver messages or documents
a baseball player on the team at bat who is on base (or attempting to reach a base)
a horizontal branch from the base of plant that produces new plants from buds at its tips
a trained athlete who competes in foot races
(football) the player who is carrying (and trying to advance) the ball on an offensive play
a long narrow carpet
device consisting of the parts on which something can slide along
fish of western Atlantic: Cape Cod to Brazil

Also: moon curser, moon-curser, base runner, ball carrier, blue runner, caranx crysos.